1. Field of the Invention
The present invention relates to a display device and a driving method thereof. More particularly, the present invention relates to an organic light emitting diode (OLED) display and a driving method thereof.
2. Description of the Related Art
Recently, there has been an increasing demand for lightweight and thin display devices as personal computers and televisions have been designed so as to be lightweight and thin. In response to this demand, traditional cathode ray tubes (CRT) are being replaced by a flat panel display device.
Such flat display panel display devices include a liquid crystal display (LCD), a field emission display (FED), an organic light emitting diode (OLED) display, a plasma display panel (PDP), etc.
In general, an active matrix type of flat panel display device includes a large number of pixels arranged in a matrix, and controls light intensity for each pixel in accordance with given luminance information to display images. Among them, the OLED display device displays images by electrical excitation and emission of self-emitting organic phosphors. Relative to other flat panel displays, the OLED display exhibits low power consumption, wide viewing angles, and high pixel response speeds, thus making it easier to display high quality motion pictures.
The OLED display includes an organic light emitting diode (OLED) and a thin film transistor (TFT) for driving the OLED. The TFT is classified according to the type of active layer, for example, into a polycrystalline silicon (polysilicon) TFT or an amorphous silicon (a-Si) TFT. Although the various advantages of using the polysilicon TFT has led to the widespread use of OLED displays, the polysilicon TFT fabrication process can be complex and costly. Moreover, it is difficult to obtain a large screen with such OLED displays.
In comparison to a polysilicon TFT, fewer steps are required to fabricate an a-Si TFT, and a large screen OLED display is generally easier to make. However, the threshold voltage of the a-Si TFT tends to shift as a DC voltage of both polarities continues to be applied to the a-Si TFT control terminal. This threshold voltage shift leads to a non-uniform current flowing in the OLED even if the same control voltage is applied to the TFT, resulting in degradation of picture quality in, and a shortened life span, of the OLED display.
To date, many pixel circuits have been proposed to compensate for a shift in threshold voltage, thereby preventing a degradation in picture quality. However, many of these pixel circuits require multiple TFTs, capacitors, and wiring, resulting in pixels having a low aperture ratio.
Accordingly, it is desirable to provide a display device that employs a simplified pixel circuit, minimizes the construction of the corresponding driving apparatus, and prevents a shift of the threshold voltage of an a-Si TFT, thereby preventing degradation of picture quality.